It is a worldwide trend that crude oil becomes heavier and inferior at the present time, however the need for heavy fuel oils decreases gradually whereas the need for light oils increases considerably. Therefore, many oil refining enterprises seek for the maximum conversion of residual oils into products such as automobile gasoline, diesel oil, and liquefied petroleum gas. An efficient way to achieve the above goal is hydrotreating of inferior heavy oil or residual oil, which significantly reduces the amount of impurities such as sulphur, nitrogen and metals, and also the value of carbon residue, thereby satisfying the requirement of raw materials for the normal process in a catalytic cracker.
U.S. Pat. No. 4,713,221 discloses a process based on a combination of normal hydrotreating and catalytic cracking of residual oil. In the process, catalytic cracker (including gas oil catalytic cracker and heavy oil catalytic cracker) heavy cycle oil (HCO) is recycled to residual oil hydrotreating unit and mixed with residual oil, and then is fed to the catalytic cracker (mainly gas oil catalytic cracker) for further process after hydrotreating. Such technical improvement changes products' distribution greatly compared with the normal operation mode wherein catalytic cracker is charged with hydrogenated residual oil as raw materials and HCO itself is recycled to the catalytic cracker. With regard to the example given in this patent wherein the new combination process is utilized with main operation parameters essentially similar, total conversion of catalytic cracker increases by 3 vol. %, mass yield of liquefied petroleum gas increases by 25.7%, mass yield of gasoline increases by 1.07%, mass yield of diesel oil decreases by 3.97%, mass yield of heavy cycle oil decreases by 15.61%, and mass yield of coke decreases by 5.56%.
CN1382776A discloses a combination process for hydrotreating of residual oil and catalytic cracking of heavy oil, in which heavy cycle oil produced in a catalytic cracker and clarified oil from oil slurry are mixed together as part of the feedstock for residual oil equipment, and the hydrogenated stream is recycled with other feedstock to catalytic cracker. Such a process can increase the yields of gasoline and diesel oil from the catalytic cracker.
CN1422327A discloses a process for increasing the yields of small molecule olefins and gasoline, wherein HCO produced in catalytic cracking equipment is hydrogenated or admixed with naphtha, then is introduced into an external, independent catalytic cracking equipment. It is proposed in the claimed process that re-cracking a cycle oil in an external second riser catalytic cracking reactor can suppress the undesirable reaction of hydrogen transfer which would occur if the cycle oil were re-cracked in a single riser reactor with other feedstock. It is beneficial to further increase the yield of light olefins. Based on the above patent, the Chinese patent CN1423689A also discloses that the catalytic cracking catalyst of meso-porous molecular sieve having ZSM-5 structure employed in an external, independent second catalytic cracking reactor may further increase the yield of light olefins. Based on the Chinese patent CN1422327A, the Chinese patent CN1425055A discloses a process for increasing the yield of light olefins by employing a composition containing different hydrogenating catalysts in a hydrogenating reactor and employing a composition containing catalytic cracking catalysts of molecular sieves with different crystal cell sizes in an external, independent second catalytic cracking reactor.
CN1262306A disclosed a combination process for hydrotreating and catalytic cracking of residual oil, which includes: introducing residual oil and clarified oil into residual oil hydrotreating equipment; hydrotreating reaction in the presence of hydrogen and hydrotreating catalyst; charging catalytic cracking equipment with the hydrogenated residual oil; conducting the cracking reaction in the presence of cracking catalyst; recycling the heavy cycle oil into the catalytic cracking equipment; separating the resulting oil slurry from the reaction via separator to obtain clarified oil; and recycling the clarified oil to hydrotreating equipment.
The yield of the products such as automobile gasoline, diesel oil and liquefied petroleum gas can be further increased by hydrotreating of catalytically cracked product heavy oils including heavy cycle oil, clarified oil or all catalytically cracked product heavy oil followed by recycling them into catalytic cracker for reprocessing. But the disadvantages in the prior art lie in the poor adjustability of products' distribution and the poor selectivity of gasoline or diesel oil in products' distribution.